Sitting in a plastic house on a 95 degree day? Yeah, you and your plants are gonna be hot! The air is going to be stifling and the temperature is going to be much hotter than 95. And we know that temps are rising into the triple digits more frequently than ever, so let’s talk about a few ways to help lower it.
Note: If you’re located in a climate that stays super hot for long periods of time, you may want to consider a shade house (without plastic) for the super hot months and a greenhouse for the rest of the year.
While these three things are written with cattle panel greenhouses and hoop houses in mind, the ideas apply to all types of greenhouses.
The first (and most fun) is to add shade. I know, it doesn’t sound especially fun - but, if your greenhouse is also a hangout space, this is a big way to create an aesthetic or mood and to decorate the space while helping your plants.
Shade will lower the temperature several degrees (usually up to 10, but sometimes more) while also making it more pleasant for you to spend time in it. There are lots of options and each has it’s pros and cons.
I use burlap for shade in my greenhouse and it creates a billowy, elegant look (I know we don’t usually associate burlap with elegant, but it works!). My friend (whose greenhouse inspired mine) uses large pieces of overlapping lace in shades of white and cream. It creates a beautiful, dreamy cottage look, and glows at night with fairy lights. Of course, there is also garden shade cloth which comes in various colors, and creates a clean, traditional greenhouse look.
The biggest decision in deciding what to use is knowing how much light you want to come through the shade cloth. I get more detailed below about different shade percentages, but know that 50% shade will work beautifully for a wide variety of plants, flowers, and herbs.
Heat-loving fruits and vegetables such as tomatoes, cucumbers, and strawberries, generally need shade cloth that is around 30% shade, meaning that about 70% of the sunlight gets through to the plants
Flowering plants and many fruits and vegetables generally like 40%-50% shade
Partial-sun vegetables and herbs such as lettuce, spinach, and cilantro do well in 60% shade
Shade loving plants like ferns, orchids, and philodendrons like 60%-90% shade
People and animals will be the most comfortable in 80%-90% shade
Shade cloth made specifically for gardening will be labeled with a shade percentage so that you can choose the best one for your plants.
Burlap is loosely woven and is a great option that provides about 50% shade.
I don’t have actual numbers on lace, but I’m going to guess it provides 30%-50% shade. It will depend on how open the lace pattern is, and whether multiple pieces are layered on top of each other.
I’ve seen some folks use pretty sheets as shade cloth. I’m going to guess that they will have a fairly high shade percentage since the fabric is tightly woven.
If you want to know more about shade cloth including the difference in colors, woven vs knit, and inside vs outside the greenhouse, check out my post How to Choose the Right Shade Cloth for Your Greenhouse.
The second way to lower the temperature in your greenhouse is through ventilation. As the air heats up, we want to let it escape or push it out. Hot air rises, and glass and polycarbonate greenhouses have roof vents that allow the air to leave through the top. It’s a little more difficult in plastic greenhouses since they don’t have roof vents. Here are some ways to ventilate cattle panel and hoop greenhouses:
Consider building it with 2 doors, one on each end. On high temperature days, you’ll want to leave them open all day. If you don’t have a second door and can’t easily add one, I would install a large window opposite of the door. Your plants will benefit from air flowing through the greenhouse, allowing for an exchange of air between the inside and the outside.
Add extra windows and screens on the ends of your greenhouse. Many cattle panel greenhouses are built with a wood frame on the ends. These are ideal places for simple expandable window screens or magnetic window screens. They are easy to install and easily covered with plastic during the winter. Unlike the doors, the side screens have the benefit of being directly in front of the plants.
Roll up the bottom edge of the plastic on the sides of the greenhouse. This option will depend on how your greenhouse is built. On my greenhouse I built the side walls to have a 1” space between the horizontal boards. In the summer, I roll up the plastic and this allows for additional air exchange. In the winter I roll it back down and set heavy pots on it to keep it in place.
Install a solar powered exhaust fan. As of this writing, I haven’t tried one of these, but I’m strongly considering it. They are designed for chicken coops, greenhouses, sheds, and the like. The ideal location would be to mount it above the door frame since that’s the highest point. My greenhouse design doesn’t have enough room for it there, but I could place it fairly high next to the door frame (after I add some wood to secure it to).
And the third way to lower the temperature is to run a fan. Once you’ve got ventilation, a fan will push air through the greenhouse and move that hot, stagnant air along. Most greenhouse experts will say that a fan is a must have and I agree.
Not only will it help lower the temperature, but air movement is important to help keep plants healthy. Circulating air helps to prevent damp conditions which lead to fungal growth and rot on plants. Damp conditions also attract slugs and gnats. Another benefit of moving air is that it helps plants grow stronger limbs as they sway in the breeze.
When purchasing a fan, you’ll want to be sure it’s rated for outdoor, all weather use. These are a little pricier than indoor fans, but they are safer and will last longer. The electrical components are sealed and protected. Be sure to power it with an outdoor-rated extension cord. In my case, my greenhouse is very close to my garage, which has power. I run an extension cord between the door and the door frame of the greenhouse.
Most outdoor fans can be mounted to the wall or set on a surface. Mine is sitting on my potting bench in the corner of the greenhouse. The diameter is 14” and it does a nice job of circulating the air. There are quite a few days where I turn it on in the morning and let it run all day. As an added convenience, I use a smart plug in my garage outlet. This allows me to turn on the fan (or the lights) with an app on my phone.
Shade, ventilation, and a circulating fan are all the ways I lower the temperature in my cattle panel greenhouse. What do you do? I’d love to hear about your methods!
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